The present invention relates to a multicarrier receiver for receiving a sequence of cyclically extended multicarrier symbols.
Such a multicarrier receiver is already known in the art, e.g. from the article xe2x80x98A Multicarrier E1-HDSL Transceiver System with Coded Modulationxe2x80x99 from the authors Peter S. Chow, Naofol Al-Dhohir, John M. Cioffi and John A. C. Bingham. This article was published in Vol. 4, No. 3, May-June 1993 of the Journal of European Transodions on Telecommunications and Related Technologies (ETT), pages 257-266. Therein, FIG. 5 represents a block scheme of a multicarrier receiver which is able to receive a sequence of cyclically extended multicarrier symbols, the so called discrete multi-tone (DMT) symbols. The effect of intersymbol interference due to transmission of the DMT symbols over a channel between multicarrier transmitter and multicarrier receiver can be removed by adding a cyclic extension to each DMT symbol with a length superior to the channel impulse response length. The data rote however reduces linearly proportionally to the length of the cyclic prefix that is added to the DMT symbols so that the length of the cyclic extension of DMT symbols has to be limited to an acceptable number. If the channel impulse response is larger than the cyclic extension, remaining intersymbol interference (ISI) will depend from the part of the impulse response exceeding the cyclic extension length. To compensate for this remaining intersymbol interference (ISI), the received DMT symbols are equalised by a time domain equaliser TEQ which is an adaptive traditional linear equaliser that allows to reduce the length of the cyclic prefix of DMT symbols to an acceptable number of bits by flattening the transmission line impulse response. After being equalised in the time domain, the DMT symbols are serial-to-parallel converted, their cyclic extension is removed, and the non extended DMT symbols are applied to the input of a fast Fourier transformer FFT which demodulates the DMT symbols by converting the symbols from time domain to frequency domain.
Although different carriers or tones may be affected differently when transmitted over the channel, the time domain equaliser proposed by Peter S. Chow et al. in the above mentioned article equalises all carriers of the multicarrier symbol in the same way and as a result limits the performance of the multicarrier system unduly. Indeed, since the known equaliser cannot be optimised individually per carrier, this equaliser is not able to fully optimise the capacity of the system. Carriers which are more affected than others for instance are not equalised more intensively. If for instance part of the carriers are unused, equalisation thereof, although not necessary, is not avoided. The equalisation is performed for all carriers, because of the structure, and by no means it is possible not to equalise groups of carriers, for example the unused ones. Equivalently, in the known system the equalisation complexity does not reduce if part of the carriers ore unused and the equalisation effort cannot be concentrated on equalisation of the more affected carriers with as consequence that the performance of the known system is not fully optimised.
An object of the present invention is to provide a multicarrier receiver of the above known type, but whose performance is increased whilst its complexity is kept at the some level, or even smaller levels.
According to the invention, this object is achieved by the multicarrier receiver defined in claim 1.
In this way, by replacing the known time domain equaliser with a per carrier frequency domain equaliser acting on the output of a sliding Fourier transformer, channel equalisation for one carrier is made independent from channel equalisation for the other carriers. The taps for equalisation of a carrier can be set independently from the tap settings for equalisation of other carriers so that performance can be individually optimised per carrier. Furthermore, if the tapped delay lines used for equalisation of more affected carriers include more taps than tapped delay lines used for less affected carriers, equalisation effort is concentrated on the most affected carriers. In particular, if the number of taps in a tapped delay lines used for equalisation of an unused carrier is made zero, no effort is wasted to equalise such an unused carrier.
It is remarked that although at first glance, replacing the fast Fourier transformer of the known multicarrier receiver with a sliding Fourier transformer significantly increases the complexity of the multicarrier receiver, efficient implementation of this sliding Fourier transformer, increases the complexity only in a negligible way, or even allow smaller complexities as it is explained further in this application.
It is to be noticed that the term xe2x80x98comprisingxe2x80x99, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression xe2x80x98a device comprising means A and Bxe2x80x99 should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. In this respect, it is noticed for instance that the multicarrier receiver according to the present invention may be equipped with a windowing unit as described in the European Patent Application EP 0 802 649, entitled xe2x80x98Method and windowing unit to reduce leakage, Fourier transformer and DMT modem wherein the unit is usedxe2x80x99.
Similarly, it is to be noticed that the term xe2x80x98coupledxe2x80x99, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression xe2x80x98a device A coupled to a device Bxe2x80x99 should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.
An additional feature of the multicarrier receiver according to the present invention is defined in claim 2.
Thus, the gain in performance compared to the known system is even more increased in case the per-carrier frequency domain equaliser is provided with adaptive taps. Indeed, whereas in an adoptive version of the known multicarrier receiver, adaptation of the equaliser taps inevitably had an influence on all carriers, the taps of a tapped delay line equalising one carrier according to the present invention can be adapted independently from the taps of other tapped delay lines equalising other carriers.
A further feature of the present invention is defined in claim 3.
Hence, in a preferred embodiment of the invention, the signal to noise ratio for transmission of multicarrier symbols over a channel between a multicarrier transmitter and the multicarrier receiver of the present invention is maximised via a mean square error criterion which allows to determine the complex tap values of the per-carrier frequency domain equaliser in which an error function expressing the mean squared difference between received and expected carrier""s QAM (Quadrature Amplitude Modulation) symbols is minimised.
Another advantageous feature of the multicarrier receiver according to the present invention is defined in claim 4.
In this way, the gain in performance compared to the known system is yet more increased, and the search for the optimal equaliser is simplified. Indeed, whereas in an advanced version of the known mulicarrier receiver, the number of equaliser taps is adjustable for all carriers in the same way, the number of taps of the tapped delay lines in the present multicarrier receiver can be increased or decreased independently from each other so that equalising effort can be concentrated on the most affected carriers whilst less affected carriers or unused carriers can be equalised slightly. If a carrier suddenly becomes more affected, the number of taps in the tapped delay line associated with this carrier is increased to improve equalisation and the complex tap values of the enlarged tapped delay line are re-calculated to optimally compensate for the increased noise. Similarly, if noise affecting a carrier suddenly decreases or if a carrier is less intensively used because less data bits are allocated thereto for instance, the number of taps in the tapped delay line associated with this carrier is decreased and the complex tap values for the remaining taps are re-calculated so that the remaining taps allow a sufficient resistance with respect to the noise.
Also an advantageous feature is defined by claim 5.
In this way, by allowing different individual delays for different carriers, the number of taps in the tapped delay lines can be decreased thus reducing the total complexity of the multicarrier receiver.
Furthermore, an advantageous feature of the present invention is defined in claim 6.
Indeed, if it is assumed that the first and second part that have to be Fourier transformed respectively start with the first and second sample of a received extended multicarrier symbol, then this second part only differ from the first part in that it does not contain the first sample thereof and in that its last sample was not contained by the first part. Since the Fourier transformation is a linear operation, the contribution of the first sample of the first part to the Fourier transform of the first part can be subtracted from this Fourier transform of the first part and a contribution of the last sample of the second part can be added to the Fourier transform of the first part to obtain the Fourier transform of the second part. If this principle is incrementally repeated, the sliding Fourier transformer has to calculate only one full Fourier transform per received multicarrier symbol and can determine all other Fourier transforms with negligible effort (only additions and subtractions).
Still another feature of the present multicarrier receiver is defined in claim 7.
In this way, the sliding Fourier transformer is implemented by a traditional Fourier transformer, preferably applying the fast Fourier transform algorithm, and some complexity in the per-carrier frequency domain equaliser whose taps now contain a first contribution for equalisation of the channel and a second contribution for achieving the sliding Fourier transformation.
Yet another advantageous feature of the present invention is defined in claim 8.
Indeed, as will be explained later on in the description, the major portion of the RLS-based computations for iterative update of the equaliser taps is common for all carriers in the embodiment of the present invention according to claim 8. As a consequence, the optimal convergence properties of the RLS based technique become achievable without unacceptable computational cost. Simulations have shown that at initialisation acceptable convergence is achieved with less than 100 training symbols when RLS based updating is used whereas LMS based initialisation of the taps of a multicarrier frequency domain equaliser requires thousands of training symbols to be processed.